Trap (plumbing)

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Water seal under a sink. Notice that siphoning takes place.

Trap with copper drain pipe at underside of firestop packing in 2 hour fire-resistance rated concrete floor slab.

Examples of traps

In plumbing, a trap is a U-, S-, or J-shaped pipe located below or within a plumbing fixture. A U-shaped trap is also known as a U-bend. It is used to prevent sewer gases from entering buildings. In refinery applications, it also prevents hydrocarbons and other dangerous gases from escaping outside through drains.

Because of its shape, the trap retains a small amount of water after the fixture's use. This water in the trap creates a water seal that prevents sewer gas from passing from the drain pipes back into the occupied space of the building. Essentially all plumbing fixtures including sinks, bathtubs, and toilets must be equipped with either an internal or external trap.

Because it is a localized low-point in the plumbing, sink traps also tend to capture heavy objects (such as jewelry) that are inadvertently dropped into the sink. Traps also tend to collect hair, sand, and other debris and limit the ultimate size of objects that will pass on into the rest of the plumbing, thereby catching over-sized objects. For all of these reasons, most traps can either be disassembled for cleaning or they provide some sort of cleanout feature. For example, the image to the right shows a removable drain screw at the bottom of each trap.

1 Venting and auxiliary devices
2 Accepted traps
3 See also
4 External links

[edit] Venting and auxiliary devices

Maintaining the water seal is thus critical; traps can and do dry out, and poor venting can siphon or blow water out of the traps. This is usually avoided by venting the drain pipes downstream of the trap; by being vented to the atmosphere outside the building, the drain lines never operate at a pressure much higher or lower than atmospheric pressure. Plumbing codes usually provide strict limitations on how far a trap may be located from the nearest vent stack.

When a vent cannot be provided, codes may allow the use of an air admittance valve instead. These devices avoid negative pressure in the drain pipe by venting room air into the drain pipe (behind the trap). A "Chicago Loop" is another alternative.

When a trap is installed on a fixture that is not routinely used, the eventual evaporation of the water in the trap must be considered. In these cases, a trap primer may be installed; these are devices that automatically recharge traps with water to maintain their water seals.

[edit] Accepted traps

In many locations, "S" traps are no longer accepted by the plumbing codes as these traps tend to easily siphon dry even when well-vented. Certain drum-styled traps are also discouraged or banned.